EXPERT GARDENING ADVICE
There a several ways to heat a small greenhouse with forced hot air being the most popular. You can buy a small heater from $20 up to $250 for a small greenhouse. I used a $20 (use at your own risk), 1500-watt heater and it kept things alive on a 6-degree f morning. They make actual greenhouse 1500-watt heater and it is about $120. The difference is the cheaper model is not designed for a greenhouse where the more expensive model is semi-waterproof and intended to be used outdoors. It is imperative that you use a GFI outlet in your greenhouse especially if you plan to use a heater not designed for outdoor use to avoid electrocution.
There are drawbacks to using hot air heating:
I use a Lux WIN100 thermostat outlet for my eletric space heater. The heating unit has a built in thermostat by to keep a consistant temperture, you need to use an outlet thermostat.
This thermostat has a hot and cold setting. It could be used for a space heater in the winter and used with an exhaust fan in the summer months.
The bench heater is controlled by an Inkbird Dual Stage Digital Temperature Controller. We use hot water to heat our bench and the Inkbird controls the circulator pump. It could also control a large heating pad or any device less than 1200 Watts. The Inkbird has a temperature probe that is inserted into soil of one of the plants and controls the temperature within 2 degrees.
An alternative to hot air heat is the use of a heated bench. I use this method to heat my greenhouse. It cost about $350 to install with copper pipe but you’re only heating the plants not the greenhouse. I use a small electric hot water heater, a circulator pump controlled by a thermostat and 70 feet of half inch copper tubing on an 80 X 24-inch bench. I also use automotive antifreeze to prevent it from freezing when not in use. The hot water heater is set for 120 degrees which provides a 75-degree soil temperature and an ambient temperature of approximately 65-70 degrees around the plants. It also is enough to keep the rest of the greenhouse from freezing. The thermostat shuts down the circulator pump when the soil reaches 75 degrees. Not only does it cost less to operate, it does not dry out the plants. I also find the plants are healthier after moving them outdoors as they become acclimated to cooler air temperatures at night.
I once saw a guy use a heater from an old pop-up camper. It had outside ventilation and did an exceptionally good job keeping his plants from freezing. Also, since the power source is 12-volt, it was off the grid. He used a deep cycle battery for power and used a 100-watt solar panel to keep it charged. His greenhouse was a Harbor Freight 10X12 and he could maintain any temperature desired. He used one 20lb barbeque each week on cold weeks.